There are many applications for lasers that require the output beam of a laser to be optically isolated to prevent back reflections damaging the laser or causing undesirable optical interactions. Examples include welding, cutting, drilling, cladding, brazing, marking, engraving, and slicing materials, especially highly reflective materials such as copper, brass, gold, silver and diamonds. The optical isolation is typically performed using an optical isolator through which the output beam of the laser is coupled. Back reflected light is then prevented from returning to the laser by the isolator. For low power laser systems, it is usually satisfactory to collimate the laser beam through the isolator. The laser beam would then be expanded by a beam expanding telescope and then coupled into a mechanical scanning optics which directs the beam to the material to be processed.
Mechanical scanning optics come in standard formats each of which is optimized for a collimated light beam of a specific beam diameter. Beam diameters (1/e2) of 5 mm, 7.5 mm and 10 mm are very common choices. The laser manufacturer thus needs to provide beam expanding optics, such as beam expanding telescopes (BET), that output collimated laser beams that have these beam diameters.
As the average power of the laser system increases above approximately 1 W, the size of the collimator generally increases because of the need to avoid laser induced damage at optical surfaces of the crystals within the isolator. If size, cost (of an individual isolator) and isolation performance is not critical, the laser beam can still be collimated through the isolator, and expanded with beam expanding telescopes.
However, if cost is an issue (for example in lasers manufactured in high volumes for consumer electronics and marking applications), then isolator crystal size must be kept to a minimum. This leads to a requirement to focus the laser beam through the isolator in order to optimize the isolation performance. The light beam emerging from the isolator is therefore not collimated. A conventional approach would be to provide a collimating lens to collimate the light from the isolator to a standard beam diameter which is compatible with standard beam expanding telescopes. The benefits of this approach is that it reduces design effort as standard optics can be used.
The situation becomes more complicated when the same isolator product is being used with a family of high power lasers, where each laser type within the family has a different beam quality. This is because the optimization of the beam profile through the isolator is usually different for each laser, and in addition, different collimating lenses must be selected to collimate the output light to be compatible with the standard beam expanding telescopes.
Beam quality is usually defined in terms of M2 or the beam parameter product. The M2 of a laser beam defines how rapidly the beam diverges from a given aperture compared to a Gaussian beam from the same aperture: an M2 of 2 means that the beam diverges twice as fast, and an M2 of 3 three times as fast. The beam parameter product (BPP) is related to the M2 by the equation:BPP=M2·λ/π(mm.mrad)where the wavelength λ is given in micrometers (μm). Thus a ytterbium-doped fiber laser emitting at 1.06 μm and with an M2=1.0 would have a BPP=0.338 mm.mrad. If the laser had an M2=2, then the BPP=0.672 mm.mrad, and if the laser had an M2=3, the BPP=1.01 mm.mrad.
If a product family of lasers were designed comprising three lasers having M2=1, M2=2, and M2=3, then the difference in the beam quality (and hence beam divergence) would result in three different beam optimizations being required at the input to the respective optical isolator. There would also be three different beam collimator designs required for each of the three fiber lasers in order to match the output beams to the mechanical scanners described previously. These nine different collimator designs would not be interchangeable between the lasers without there being additional functionality to adjust the output beam to provide a collimated beam matched to the individual mechanical scanner units. Each of the lasers would have a single collimator design at its output to match the output beams to the expanded beam telescopes.
However, this conventional approach is not the option with the lowest cost materials. This is because the collimating lens at the output of the isolator is serving a similar purpose as the first lens in the beam expanding telescope. There is therefore a redundant optical element within the system. Although acceptable for systems produced in low quantities where the additional expense of designing special optics is more than the cost of using standard beam expanding telescopes, it is not the optimum solution for a high volume product where cost and space is at a premium.
There is therefore a need for an isolator design that removes the additional collimating lens at the output of the isolator. Unfortunately, removal of this lens causes a major design problem for a product family of lasers defined by different beam qualities. In the example quoted above, there would need to be three different beam expanding telescopes for each of the three lasers. These beam expanding telescope designs would not be interchangeable between the lasers without additional functionality such as additional lenses or adjustability) being provided. Thus there is a need to provide nine different beam expanding telescopes in order to match the beams to the three mechanical scanner units. The complication described above increases rapidly with the number of different laser beam qualities and the number of mechanical scanner units being catered for. And it is for this reason that prior art laser systems for high volume requirements are provided with a range of beam adaption accessories, which need to be designed, manufactured and supported in customer applications
It would be advantageous if there were optical isolators and laser systems that enable greater commonality of laser accessories between different laser types. Such an advance would lead to fewer accessories that need to be designed, manufactured and controlled by the laser user. An aim of the present invention is to provide an apparatus and method for optical isolation which reduces the above aforementioned problem.